Various methods for effecting purification of gases containing solvent vapours by adsorption have been devised. As occasion requires, these prior methods involve recovery of the removed solvent vapours. Of these prior methods, a popular method makes use of the so-called fluidized-bed type adsorption system wherein a gas to be treated and adsorbent particles such as activated carbon, activated alumina or silica are brought into mutual contact to form a fluidized bed of the adsorbent particles. In the adsorption treatment of the gas by this fluidized-bed method, it is common practice to effect the gas treatment continuously by having fluidized beds formed in a multiplicity of stages within a tower as illustrated in FIG. 1 of the accompanying drawing, for example. In FIG. 1, 1 denotes a reaction tower. A gas containing solvent vapours to be removed is introduced into the tower 1 through a nozzle 2 in the adsorption section A. On entering the tower interior, the gas ascends vertically and comes into contact with adsorbent particles held inside the adsorption section A, causing the adsorbent particles to form fluidized beds on the stepped trays 3, 3', 3" . . . The adsorbent particles forming the fluidized beds adsorb the solvent vapours from the gas. The gas which has thus been freed of the solvent vapours is released into the atmosphere via a discharge outlet 4 at the top of the tower 1. The adsorbent particles on the stepped trays 3, 3', 3" . . . , fall through the downcommers 5, 5', 5" . . . associated with the trays and descend gradually downwardly by virtue of gravity, while simultaneously adsorbing the solvent vapours from the gas. Then, they leave the adsorption section A and accumulate in the space formed on a partitioning plate 6. While they form a gravitationally moving bed in the space, they gradually reach a regeneration section B which is located at the bottom of the reaction tower 1. On entering the regeneration section B, the adsorbent particles are heated by a heater 7, with the result that the particles are regenerated as they are forced by the heating to release the adsorbed solvent vapours. Subsequently, the regenerated adsorbent particles reaching the bottom 8 of the tower 1 are transferred via a lifting pipe 9 to the top of the tower 1 for recyclic service. In the meantime, the solvent vapours which have been desorbed from the adsorbent particles are forced out of the system via a nozzle 10 by means of a carrier gas introduced via a nozzle 11 disposed in the lower portion of the regeneration section B. The discharged solvent vapours are transferred to a desorbate treating section C composed of a condenser, decanter and the like.
In the afore-mentioned fluidized-bed type gas treatment methods, steam gas is usually employed as the carrier gas for regenerating the adsorbent particles. The steam gas serving as the carrier gas is a useful gas since it can effectively make the adsorbent particles regain the adsorbing power and is also inexpensive. However, when the adsorbent particles containing the adsorbed solvent vapours are subjected to a regeneration treatment by using steam gas as the carrier gas and simultaneously the solvent vapours are subjected to a recovery treatment, the recovered solvent is adulterated with water from condensation of the steam, if the solvent is an organic solvent such as alcohol or ketone, which has compatibility with water. In order to effectively utilize the recovered organic solvent, therefore, it is necessary to separate and remove the water from the recovered solvent. For this purpose, an additional separating unit is required.
As a remedy for the above-mentioned defect arising when a condensable gas such as steam is used as the carrier gas for regenerating the adsorbent particles, the use of an incondensable inert gas such as nitrogen gas is considered to be good. However, in case such an incondensable inert gas is employed as the carrier gas, it must be used in a recyclic way. This is because the incondensable inert gas in general is too expensive to release into atmosphere after use, unlike the case of the steam. The recyclical use of the incondensable inert gas is generally unadvisable. That is, the incondensable inert gas introduced into the regeneration section B (See FIG. 1) comes into contact with the adsorbent particles therein to desorb the solvent vapours from the particles containing the adsorbed solvent vapours, and subsequently the gas accompanied by the solvent vapours thus desorbed is led to the desorbate treating section C (See FIG. 1). The incondensable inert gas thus introduced into the desorbate treating section C is cooled in a condenser to discharge the solvent vapours in the form of liquid, but in this case, the solvent vapours in an amount corresponding to the vapour pressure at the cooling temperature inevitably remain in the incondensable inert gas without being discharged. Accordingly, the incondensable inert gas leaving the desorbate treating section C cannot be introduced into the regeneration section B again as is because the adsorbent particles could not be fully regenerated. For this reason, the remaining solvent vapours must be separated and removed from the gas by means of an additional separating unit prior to the gas being introduced into the regeneration section B. Thus, when the incondensable inert gas is recyclically used, the apparatus and the operation become greatly complicated. Therefore, at the present time, incondensable inert gases are not substantially used as the carrier gas.